There's a possibility that you may not be getting the highest "quality" yorkie, but at least you're not paying top price.
Like others have sd., his appearance at this age gives me a bit of pause too. I would just keep communicating with the breeder in a positive way, so that she doesn't get turned off by you, but is aware that it's a concern of yours. Let her know you love him and really want him, but when you're spending that much money, you also need to feel assured that you're getting what you're paying for. Jokingly blame it on the husband or something
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What age are you supposed to bring him home at? Because his appearance is so non-standard, I'd personally want to at least ask if I could leave him with her as long as possible, and then maybe get my money back if he didn't start fluffing out a bit soon. I know this would be very hard for you, as in the meantime you're falling in love with him.....but maybe she would offer to come down on her price if he continues to look "different" than he should?
Having said all that....when I got my Kiki, I also pd. $800, and also got a puppy that looked a little different from the rest of her litter. For awhile, people would ask me if she was a yorkie-poo...she had VERY fluffy hair, *except* on her face....while yorkie pups usually have the longer muzzle hair, hers was shorter, and very slow to grow in. I never minded people asking if she was a mix, because I knew that she wasn't, but that she did look like it!
And guess what....she eventually grew out of it! Now she isn't the best example of a yorkie according to the "standard", but she looks just like many of the other light-haired, cotton-coated, floppy-earred cuties that we see on this site!
Another thing possibly in your puppy's favor....I notice it looks like he has a bit of white on his chest? I've read that pups with that usually end up having nice silky hair. And while I've never seen one as "smooth" as he is, yes, I've read that often the silky hair takes the longest to grow in.
So I think you're right to be concerned, and ask questions....and also that you're right to not completely discount him. I'd just continue to stay on top the situation like you are. Let us know how it turns out!